1. Field of the Invention
The present invention relates to a rotating stirring device particularly for mixing heterogeneous systems in mechanical stirred reactors.
2. Description of the Related Art
Various types of mechanical stirred reactors are known in the art, generally of cylindric shape, in which processes are carried out where a continuous and uniform mixing of various components and an efficient mass exchange between different phases (for instance, gas-liquid, gas-liquid-solid, liquid-solid, liquid-liquid, etc.) are required. The mechanical stirring is commonly carried out through a stirrer put inside the reactor in an axial position. The particular stirrer shape (turbine, blade, propeller, etc.) is mainly chosen according to the viscosity of the mass to be stirred (see "Advances in Chemical Engineering", vol. 17, pag. 5-8, Academic Press, 1992).
A common shortcoming of prior art stirring devices is in the case of gas-liquid systems, a generally low mass transfer coefficient between the phases which considerably decreases when the volume of the mass put under stirring is increased with time. This occurs, for instance, in polymerization reactions in emulsion or in suspension in which at least one of the monomers is present in the gas phase. During the polymerization a gradual increase in the volume and the solid content of the liquid phase occurs. A lowering of the mass transfer between the gas phase and liquid phase and consequently a reduction of the process yield, results therefrom.
The difficulty in obtaining satisfactory mass exchanges between the various phases imposes limits in the reactor shape. In particular, with the traditional stirring systems the use of reactors with a large height/diameter ratio, which would yield a more efficient thermal exchange, is impossible.
In gas-liquid systems, in order to improve the mass exchange between the phases, devices which distribute the gas inside the liquid phase (spargers) are used, they are in practice, toroidal shaped devices, with holes through which the gas is bubbled. Such devices require very frequent maintenance, since they tend to be easily clogged, especially when in the liquid phase a solid phase is present or is formed during the reaction (for example, in polymerization reactions).
The Applicant has now invented a new type of rotating stirring device as described hereinunder, particularly suited for mixing heterogeneous systems, which permits a very efficient mass exchange between different phases to be obtained and which remains substantially constant when the volume of mass stirred increases. Moreover a high mixing homogeneity, with particularly low shear stress values, is achieved by the inventor.